Explanation of each option:

1. Phytoextraction

• Plants take up contaminants (mainly metals) from soil and accumulate them in harvestable shoots.

• Contaminants are removed from soil, not merely immobilized in it.

• So it does not match “complexation and immobilization within the soil.”

2. Phytodegradation

• Plants (and root-associated microbes) enzymatically break down organic pollutants into less toxic products.

• This is active chemical degradation, not just stabilization/complexation in the soil matrix.

3. Phytovolatilization

• Plants absorb contaminants, convert them to volatile forms, and release them to the atmosphere via transpiration (e.g., volatile selenium or mercury species).

• Again, the contaminant is transferred out of the soil, not immobilized there.

4. Phytostabilization – correct

• In phytostabilization, plants and their rhizosphere immobilize contaminants in the soil through adsorption, precipitation, and complexation, reducing mobility and bioavailability while leaving them in place.

• This exactly corresponds to “complexation and immobilization of toxin within the soil,” so option 4 is correct.